Well, I have had an even luckier escape! When I pulled the old expansion tank, I noted a spring and bronze valve sitting in the socket that the expansion tank plugs into. Not knowing what to expect I thought this was normal. The spring appeared to seat in the bottom of the expansion tank.

While waiting for my new expansion tank to arrive, I had a look at a few Youtube videos when I came across this:-

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While I don't necessarily agree with the guys thoughts on expansion tank failures, I could see that the spring and valve I had were supposed to be part of a larger assembly. No sign of the top part at all. When I tried to remove the lower part, the plastic disintegrated and I had one hell of a job getting the rest of it out.

The part is called the Auto Transmission Thermostat, part # 17111437362 and is $90 at Clickable. Lucas is express posting one to me so I can install it with the expansion tank.